Phase change random access memory (PCRAM) makes use of the property of the phase change material (PCM) being conductive in crystalline state and comparatively non-conductive in an amorphous state. The conductive crystalline state can be assigned a logic 1 and the non-conductive amorphous state can be assigned a logic 0 respectively. The material used typically is a chalcogenide compound, e.g. Ge2Sb2Te5 (“GST”) or an Ag—In—Sb—Te compound.
The phase change from an amorphous state, that is the comparatively non-conductive state, into the crystalline state requires heating the material for a short time above the glass transition temperature without melting the material. This can be achieved in that the material is placed between two electrodes and an appropriate heat current pulse. The heat current pulse must be strong enough to heat the material above its glass transition temperature, so that the material crystallizes.
Vice versa a phase change from the crystalline phase to an amorphous state can be achieved by heating the material for a short time beyond the melting temperature with subsequently “quenching”, that is rapidly cooling off, the material into an amorphous state, thus erasing the logic 1 to 0.
For the above mentioned Ge2Sb2Te5 (GST) the melting temperature is at around 600° C., the glass transition temperature is at around 300° C. typically and the crystallization time is around 50 ns.
To achieve a correspondingly quick heating of the switching active material beyond the crystallization or melting temperature, respectively, relatively high currents may be necessary, which may cause correspondingly high power consumption.
Furthermore, the consequence of high heating currents may be that the corresponding cell can no longer be controlled by an individual transistor with small structure size. This may result in a corresponding—possibly strongly reduced—density of elements on the respective memory device.
So far one has primarily been trying to restrict the programmed volume by a reduction of the contacting surface and to thus reduce the required currents.
Previous concepts are inter alia described in: S. J. Ahn, Y. N. Hwang et al., “Highly reliable 50 nm contact cell technology for 256 Mb PRAM”, Samsung Electronics, Symposium on VLSI Technology Digest of Technical Papers, wherein a ring-shaped contact structure is proposed so that the current flows through the perimeter of the ring-shaped contact.